Method and apparatus for preventing radio communication system access by an unauthorized modem

ABSTRACT

A method and apparatus for preventing radio communication system access by an unauthorized modem. The apparatus comprises a signal detector that determines if an authorization signal has been received from the base station within a specified period of time. The authorization signal authorizes the apparatus to communicate with the base station. A transmitter transmits information to the base station, and a controller disables the transmitter of the apparatus providing that the authorization signal has not been received within the specified period of time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to radio communication systems, and,more particularly, to a method and apparatus for preventing radiocommunication access by an unauthorized modem.

2. Description of the Related Art

Over the past several years, wireless communication devices have becomeincreasingly popular due in part to the freedom of movement they provideto their users. Wireless communication devices today are found in avariety of forms such as pagers, cellular phones, and with theincreasing need for mobile computing, portable computers equipped with awireless modem are now commonly used as mobile work stations.

Typically, telecommunications services are provided between a cellulartelecommunications network and a mobile station over an air interfaceusing radio frequencies. Each subscriber having a mobile station isassigned a unique International Mobile Subscriber Identity (IMSI). Atany moment, an active mobile station may be in communication over theair interface with one or more base stations. The base stations are, inturn, managed by base station controllers (also known as radio networkcontrollers). A base station controller together with its base stationscomprise a base station system. The base station controllers of a basestation system are connected via control nodes to a coretelecommunications network, such as the public switched telephonenetwork (PSTN). One type of standardized mobile telecommunicationsscheme is the Global System for Mobile communications (GSM). GSMincludes standards that specify functions and interfaces for varioustypes of services. GSM systems may be used for transmitting both voiceand data signals.

A particular base station may be shared among multiple mobile stations.Because the radio spectrum is a limited resource, the bandwidth isdivided using a combination of Time-Division and Frequency-DivisionMultiple Access (TDMA/FDMA). FDMA involves dividing the maximumfrequency bandwidth (e.g., 25 MHz) into 124 carrier frequencies spaced200 kHz apart. A particular base station may be assigned one or morecarrier frequencies, and each carrier frequency is divided into timeslots. During an active session between the base station and the mobilestation, the base station assigns the mobile unit a frequency, a powerlevel, and a time slot for upstream transmissions from the mobilestation to the base station. The base station also communicates aparticular frequency and time slot for downstream transmissions from thebase station destined for the mobile station.

The fundamental unit of time defined in GSM is referred to as a burstperiod, which lasts 15/26 ms (or approx. 0.577 ms). Eight burst periodsare grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which isthe basic unit for the definition of logical channels. One physicalchannel is defined as one burst period per frame. Individual channelsare defined by the number and position of their corresponding burstperiods.

GSM frames, each frame having 8 burst periods, are grouped intosuperframes (e.g., groups of 51 frames) that include both traffic (i.e.,voice or data signals) and control information. The control informationis conveyed over common channels defined in the superframe structure.Common channels can be accessed both by idle mode and dedicated modemobile stations. The common channels are used by idle mode mobilestations to exchange signaling information for changing to dedicatedmode in response to incoming or outgoing calls. Mobile stations alreadyin the dedicated mode monitor the surrounding base stations for handoverand other information.

The common channels include:

-   -   a Broadcast Control Channel (BCCH) used to continually broadcast        information including the base station identity, frequency        allocations, and frequency-hopping sequences;    -   a Frequency Correction Channel (FCCH) and Synchronization        Channel (SCH) used to synchronize the mobile station to the time        slot structure of a cell by defining the boundaries of burst        periods, and the time slot numbering (i.e., every cell in a GSM        network broadcasts exactly one FCCH and one SCH, which are, by        definition, sent on time slot number 0 within a TDMA frame);    -   a Random Access Channel (RACH) used by the mobile station to        request access to the network;    -   a Paging Channel (PCH) used to alert the mobile station of an        incoming call; and    -   an Access Grant Channel (AGCH) used to allocate a Stand-alone        Dedicated Control Channel (SDCCH) to a mobile station for        signaling (i.e., to obtain a dedicated channel) following a        request on the RACH.

For security reasons, GSM data is transmitted in an encrypted form.Because a wireless medium can be accessed by anyone, authentication is asignificant element of a mobile network. Authentication involves boththe mobile station and the base station. A Subscriber IdentificationModule (SIM) card is installed in each mobile station. Each subscriberis assigned a secret key. One copy of the secret key is stored in theSIM card, and another copy is stored in a protected database on thecommunications network that may be accessed by the base station. Duringan authentication event, the base station generates a random number thatit sends to the mobile station. The mobile station uses a random number,in conjunction with the secret key and a ciphering algorithm (e.g., A3),to generate a signed response that is sent back to the base station. Ifthe signed response sent by the mobile station matches the onecalculated by network, the subscriber is authenticated. The base stationencrypts data transmitted to the mobile station using the secret key.Similarly, the mobile station encrypts data it transmits to the basestation using the secret key. After a transmission received by themobile station is decrypted, various control information, including theassigned power level, frequency, and time slot for a particular mobilestation may be determined by the mobile station.

Generally, communication systems are described in terms of layers. Thefirst layer, responsible for the actual transmission of a data-carryingsignal across the transmission medium, is referred to as the physicallayer (i.e., a hardware component). The physical layer groups digitaldata and generates a modulated waveform based on the data in accordancewith the particular transmission scheme. In GSM, the physical layergenerates the transmission waveform, and transmits during the assignedtransmit time slot of the mobile station. Similarly, the receivingportion of the physical layer identifies data destined for the mobilestation during the assigned receipt time slot.

The second layer, referred to as a protocol layer (i.e., a softwarecomponent), processes digital data received by the physical layer toidentify information contained therein. For example, in a GSM system,decryption of the data is a protocol layer function. Notice that changesin the operating parameters of the physical layer are identified onlyafter decryption and processing by the protocol layer. Although thisparticular interdependency does not generally cause a problem in apurely hardware implementation, it may cause a problem when all orportions of the protocol layer are implemented in software.

Certain computer systems, especially portable notebook computers, may beequipped with wireless modems. One trend in modern technology involvesthe use of software modems that implement some of the real-timefunctions of traditional hardware modems using software routines.Because the hardware complexity of a software modem is less than ahardware counterpart, it is generally less expensive as well as moreflexible. For example, the protocol layer decryption and processing maybe implemented partially or entirely with software.

Software systems, such as PC systems, run interface control software inoperating systems environments as software drivers. These drivers areresponsible for communicating to the hardware devices and operate at aprivileged level in the operating system. Other software applicationsare precluded from affecting the drivers. However, because drivers arenot protected from other drivers, a variety of problems can occur thatmight affect the operation of a driver, such as by corrupting itsoperation. These effects may be caused accidentally, or may be caused bypurposeful hacking. A corrupted (or co-opted) driver might causeadditional problems outside the computer, such as causing a phone lineor wireless channel to be used, operating an external peripheral, ordeleting important data.

Because the operating parameters of the physical layer, which controlthe operation of the transmitter of the mobile station, are controlledby the protocol layer using software, it may be possible for a computerprogram or virus to take control of the mobile station and cause it toaccidentally or purposefully transmit outside of its assigned time slotfrequency and/or power level. A wireless communications network, such asa cellular network, relies on a shared infrastructure. A mobile stationmust adhere to the ‘rules of the road’ or it may cause interference onthe network.

If certain functions of the mobile station are controlled in software, aprogrammer may determine how the GSM control frames are decoded and howthe transmitter module is triggered. A virus may then be written andspread over the network to infiltrate the software-based mobilestations. Then, on a particular time and date, the virus could takedirect control of the mobile station and transmit continuously orintermittently and inundate the base stations and other mobile unitswith random frequencies and full power. Such a virus design could enableand disable at random times to avoid detection, robbing the air-timesupplier of some or all of his available bandwidth and may even cause acomplete shutdown of the network. Such an attack may take only a fewaffected devices (i.e., as few as one) per cell to disable the cellcompletely.

The security problems associated with mobile stations operating in ashared infrastructure may be segregated into three levels of severity:tamper-proof, non-tamperproof, and class break. First, ahardware/firmware implementation (such as a cell-phone) is the hardestwith which to tamper, because each device must be acquired individuallyand modified (i.e., tamper-proof). On the other hand, a software-basedsolution is easier to tamper with, as a hacker can concentrate on asoftware-only debugger environment (i.e., non-tamper-proof). Finally, asystem with the ability to be tampered with that is similar on allsystems and allows the tampering to be distributed to a large number ofsystems of the same type is susceptible to a ‘class-break.’

A software wireless modem is susceptible not only to a class-break, butalso it is among those devices whose code may be accessed from the samelayer as IP (internet protocol) or another portable code accessmechanism. Many software wireless modems may be integrated intocomputers coupled to networks or the Internet. Such an arrangementincreases the susceptibility of the software to being tampered with andcontrolled.

The present invention is directed to overcoming, or at least reducingthe effects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

One aspect of the present invention is seen in a device forcommunicating with a base station of a communication system. The devicecomprises a signal detector that determines if an authorization signalhas been received from the base station within a specified period oftime. The authorization signal authorizes the device to communicate withthe base station. A transmitter transmits information to the basestation, and a controller disables the transmitter of the deviceproviding that the authorization signal has not been received within thespecified period of time

Another aspect of the present invention is seen in a method forauthorizing a user terminal to communicate with a base station in acommunication system. The user terminal includes a transmitter fortransmitting information to the base station. The method includesdetermining if an authorization signal has been received at the userterminal within a specified period of time, the authorization signalauthorizing the user terminal to communicate with the base station; anddisabling the transmitter of the user terminal providing that theauthorization signal has not been received within the specified periodof time.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 is a simplified block diagram of a radio communications system,including a user terminal and base station, in accordance with oneembodiment of the present invention;

FIG. 2 is a simplified block diagram of a modem included within the userterminal of the communications system of FIG. 1; and

FIG. 3 is a flow diagram of a process for preventing the unauthorizeduse of the radio communication system of FIG. 1 by the user terminal.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Turning now to the drawings, and specifically referring to FIG. 1, asimplified block diagram of a radio communication system 100 is shown inaccordance with one embodiment of the present invention. The radiocommunication system 100 comprises a user terminal 110, whichcommunicates with a base station 120 over a radio communication channel130. According to one embodiment, the user terminal 110 takes the formof a mobile computing device (such as a portable computer, for example)that includes a wireless software-based modem 140 for communicating inaccordance with a particular radio communication protocol over the radiocommunication channel 130. According to one embodiment, the radiocommunication protocol operates in accordance with the Global System forMobile Communications (GSM) standard. It will be appreciated, however,that the communication protocol utilized between the user terminal 110and the base station 120 may include other types of known wirelesscommunication protocols and, thus, need not necessarily be limited tothe GSM standard. It will further be appreciated that although theinvention is described as it may be implemented in a wirelessenvironment, its application may be extended to a wired systemenvironment using software implemented communication protocols, such asV.90, ADSL, HomePNA or wireless LAN, for example. Accordingly, in thisparticular embodiment, the radio communication channel 130 issubstituted with a wired communication link.

The base station 120 may serve a plurality of subscribers via other userterminals (not shown), and may be indirectly coupled to other basestations (not shown) via a mobile switching center (not shown). Themobile switching center may further be coupled to a landline telephonenetwork (not shown) to permit data and/or voice communication betweenthe user terminal 110 and another device coupled to the landlinetelephone network.

In accordance with one embodiment of the present invention, the userterminal 110 may include a variety of computing devices, such as adesktop computer, a notebook computer, a personal digital assistant(PDA), and the like. For purposes of illustration, the user terminal 110is described as it may be implemented using a notebook computer equippedwith a software-based wireless modem 140. According to the illustratedembodiment, the modem 140 is installed as an internal component withinthe user terminal 110. It will be appreciated, however, that the modem140 may also be a physically separate component from the user terminal110 and may be coupled to the user terminal 110 via a connecting cable(not shown).

According to the illustrated embodiment, the modem 140 includes ahardware component (i.e., a physical layer) 150 that is implemented inhardware and a software component (i.e., a protocol layer) 160 that isimplemented by software. For purposes of illustration, the functions ofthe modem 140 are described as they may be implemented for a GSMcommunication protocol. It will be appreciated, however, that othertypes of wireless or wired communications protocols may be used in lieuthereof.

The hardware component 150 of the modem 140 is capable of convertingdigital signals generated at the user terminal 110 into an analogtransmit waveform for transmission over the radio communication channel130 to the base station 120. The hardware component 150 is furthercapable of converting an incoming analog received waveform over theradio communication channel 130 into digital received signals forprocessing by the software component 160. For the transmit signals, theoutput of the software component 160 is the transmit “on-air”information modulated about a zero Hz carrier (i.e., a carrierlesssignal). The hardware component 150 then mixes or upconverts thecarrierless transmit signal generated by the software component 160 inaccordance with particular transmission parameters, which may include anassigned time slot, frequency, and power level (communicated to the userterminal 110 by the base station 120) to generate the actual analogwaveform that is transmitted to the base station 120. According to theillustrated embodiment, the base station 120 also communicates time slotand frequency assignments to be used over the radio communicationchannel 130 to the user terminal 110 for incoming data transmitted bythe base station 120. The incoming analog receive waveform is sampledand downconverted based on the assigned time slot and frequencyparameters to recreate a carrierless receive waveform (i.e., modulatedabout zero Hz). The software component 160 receives the carrierlessreceive waveform from the hardware component 150 and performs basebandprocessing, decryption, and decoding to regenerate the received data.

The particular algorithms used for implementing the software-based modem140 are described by various industry standards, such as the GSMstandard, and are well known to those of ordinary skill in the art.Accordingly, for clarity and ease of illustration, such details are notdisclosed herein.

Turning now to FIG. 2, a more detailed representation of the wirelessmodem 140, included within the user terminal 110, is provided. The modem140 receives and transmits radio signals via a radio transceiver 205,which forms a part of the hardware component 150 of the modem 140. Areceiver 210 receives radio signals that are transmitted to the userterminal 110 by the base station 120. The user terminal 110 transmitsradio signals to the base station 120 via a transmitter 220. The radiosignals received from the base station 120 via the receiver 210 areprocessed through a receive data pump 225, which forwards the data fromthe received radio signal to the software component 160 of the modem 140for further processing (as was previously described). Similarly, datathat is originated at the user terminal 110 is sent from the softwarecomponent 160 to the transmitter 220 via a transmit data pump 230 fortransmission to the base station 120. Radio controller 235 receivescontrol code signals from the software component 160 and controls theoperation of the radio transceiver 205 in accordance with the controlcode signals sent from the software component 160 of the modem 140. Thecontrol code signals may include particular transmission parameters suchas the frequency, time slot and power level at which information (e.g.,data) is to be transmitted to the base station 120 over the radiocommunication channel 130.

In accordance with the illustrated embodiment, the base station 120periodically transmits an “Authorized to Transmit” signal (hereinafterreferred to as an “AT” signal) to the user terminal 110, whichauthorizes the user terminal 110 to communicate with the base station120. In one embodiment, the AT signal may be used to prevent the userterminal 110 from transmitting provided that the radio communicationsystem 100 deems that the user terminal 110's communication in thesystem 100 is unauthorized or otherwise communicating in an impropermanner. Such unauthorized or improper use of the radio communicationsystem 100 may include transmission on an unauthorized frequency, timeslot and/or power level. It will be appreciated that various otherfactors may deem the user terminal 110 as operating improperly withinthe radio communication system 100, and, thus, need not necessarily belimited to the aforementioned examples.

According to one embodiment, the AT signal issued by the base station120 is transmitted on a specific time slot, frequency or code that isassigned to a particular user terminal 120. The AT signal may beconfigured as a constant amplitude signal, a constant frequency signal,or a slow amplitude or pulse modulated, time constrained signal. The ATsignal may further take the form of a data change on a specific part ofa signal that may be easily decoded, such as the pre- or post-amble partof a specific time slot. It will be appreciated that the AT signaltransmitted from the base station 120 to the user terminal 110 may takeon several other forms, and, thus, need not necessarily be limited tothe aforementioned examples. In any event, however, when selecting aparticular form for the AT signal, various factors such as hardware costfor detecting the signal at the modem 140 and network capacityconstraints on the system 100, for example, should be considered.

According to one embodiment, the AT signal is transmitted from the basestation 120 to the user terminal 110 on a periodic basis, which may beevery ten seconds, for example. It will be appreciated, however, thatthe rate of recurrence for transmitting the AT signal by the basestation 120 may be more or less frequent depending on factors such assystem capacity constraints, for example.

In accordance with one embodiment, the hardware component 150 of thewireless modem 140 comprises a signal detector 240 to detect the ATsignal received by the user terminal 110 via its receiver 210. Accordingto the illustrated embodiment, the AT signal is configured to cause themodem 140 from not shutting down or disabling its transmitter 220,thereby permitting communication with the base station 120.

The hardware component 150 of the modem 140 is further configured with a“watchdog” timer 245. According to one embodiment, if the timer 245times out after a specified period of time, the timer 245 is configuredto send a “shutdown” signal to the radio controller 235 to disable thetransmitter 220, thereby preventing the user terminal 110 fromtransmitting messages to the base station 120. When the timer 245 issuesa shutdown command to the radio controller 235, the command is also sentto status block 255, where the disabling of the transmitter 220 iscommunicated to the software component 160 of the modem 140 to indicatethat the user terminal 110 is no longer permitted to transmit to thebase station 120.

When an AT signal is transmitted from the base station 120 and detectedby the signal detector 240, a signal is sent to the timer 245 to restartor re-initialize its clock. If the signal detector 240 does not receivean AT signal from the base station 120 by the time-out of the timer 245,a shutdown signal is sent from the timer 245 to the radio controller 235to disable the transmitter 220, thereby preventing the user terminal 110to transmit to the base station 120. In one embodiment, the transmitter220 of the user terminal 110 may be re-enabled upon receiving an ATsignal from the base station 120. In an alternative embodiment, the userterminal 110 may require some form of manipulation from a systemoperator of the radio communication system 100 to re-enable thetransmitter 220. Such manipulation may be in the form of transmitting aspecific control code either via a wired or wireless interface with theuser terminal 110.

Turning now to FIG. 3, a process 300 for preventing the unauthorized useof a radio communication system 100 by a user terminal 110 is provided.The process 300 commences at block 305 where the watchdog timer 245 ofthe user terminal 110 starts counting. The timer 245 counts for aspecified period of time, and upon expiration of the specified period oftime, the timer 245 is considered to have “timed-out.” According to theillustrated embodiment, the specified period of time for the timer 245is set to be for a longer period than the amount of time between twoconsecutive transmissions of the AT signal by the base station 120.

At block 310, the signal detector 240 of the user terminal 110determines whether an AT signal has been received from the base station120. If the AT signal has been detected by the signal detector 240, thena reset command is sent from the signal detector 240 to the timer 245,which restarts its clock to zero at block 315. The radio controller 235continues to allow the transmitter 220 of the user terminal 110 totransmit data messages to the base station at block 320, and the process300 reverts back to block 310, where it is again determined if an ATsignal (i.e., a second, subsequently transmitted AT signal) has beenreceived at the user station 110 from the base station 120.

If the signal detector 240 has not detected an AT signal from the basestation 120, it is determined at block 325 whether the watchdog timer245 has timed-out (i.e., the specified period of time set for the timerhas been exhausted). If the timer 245 has not timed-out, the radiocontroller 235 at block 330 continues to permit the transmission of datamessages from the user terminal 110 to the base station 120 via thetransmitter 220. If it is determined that the timer 245 has timed-out atblock 325, the watchdog timer 245 sends a shutdown signal to the radiocontroller 235 at block 335. The timer 245 may also send a status 255 tothe software component 160 of the modem 140 indicating that thetransmitter 220 of the user terminal 110 has or is about to be disabled.At block 340, the radio controller 235 sends a signal to the radiotransceiver 205 to disable the transmitter 220, thereby preventing theuser terminal 110 from transmitting data messages to the base station120. According to one embodiment of the present invention, thetransmitter 220 of the user terminal 110 may remain disabled until an ATsignal is detected again by the signal detector 240. In an alternativeembodiment, the transmitter 220 may remain disabled until an operator ofthe radio communication system 100 re-initializes the user terminal 110with a control code, for example. Such re-initializing of the userterminal 110 may be accomplished through a wired communication link oralternatively through the radio communication channel 130, for example.

The AT signal that is periodically transmitted by the base station 120provides authorization to the user terminal 110 to communicate in theradio communication system 100. When the radio communication system 100determines that the user terminal 110 is operating improperly within thesystem 100, it may stop transmitting the AT signal to prevent the userterminal 110 from misusing the system 100. Such misuse of the radiocommunication system 100 may be the user terminal 110 transmitting on anunauthorized frequency, time slot or power level, for example, asdetermined by the communication system 100.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. A method for authorizing a user terminal to communicate with a basestation in a communication system, the user terminal including atransmitter for transmitting information to the base station, the methodcomprising: determining if an authorization signal has been receivedfrom the base station at the user terminal within a specified period oftime for the transmission of the authorization signal, the authorizationsignal authorizing the user terminal to communicate with the basestation; and disabling the transmitter of the user terminal providingthat the authorization signal has not been received within the specifiedperiod of time.
 2. The method of claim 1, further comprising:re-enabling the transmitter of the user terminal upon receipt of theauthorization signal.
 3. The method of claim 1, wherein determining ifan authorization signal has been received at the user terminal within aspecified period of time, further comprises: starting a timer to countfor the specified period of time; and determining if the authorizationsignal has been received at the user terminal prior to the timerexpiring at the specified period of time.
 4. The method of claim 3,further comprising: receiving the authorization signal at the userterminal; restarting the timer to count for the specified period oftime; and permitting the user terminal to transmit information via thetransmitter to the base station upon receipt of the authorizationsignal.
 5. The method of claim 4, wherein permitting the user terminalto transmit information further comprises: permitting the user terminalto transmit information via the transmitter to the base station uponreceipt of the authorization signal until expiration of the specifiedperiod of time and non-receipt of a second authorization signal.
 6. Themethod of claim 3, wherein determining if the authorization signal hasbeen received at the user terminal prior to the timer expiring at thespecified period of time, further comprises: providing a signal todisable the transmitter of the user terminal providing that thespecified period of time on the timer has expired; and disabling thetransmitter of the user terminal.
 7. The method of claim 3, whereindetermining if the authorization signal has been received at the userterminal prior to the timer expiring at the specified period of time,further comprises: permitting the transmission of information from thetransmitter of the user terminal to the base station providing it isdetermined that a second authorization signal has not been received andthe specified period of time on the timer has not expired.
 8. A devicefor communicating with a base station of a communication system, thedevice comprising: a signal detector that determines if an authorizationsignal has been received from the base station at the device within aspecified period of time for the transmission of the authorizationsignal, the authorization signal authorizing the device to communicatewith the base station; a transmitter that transmits information to thebase station; and a controller that disables the transmitter of thedevice providing that the authorization signal has not been receivedwithin the specified period of time.
 9. The device of claim 8, whereinthe device comprises a modem having a software component with softwarerunning thereon and a hardware component that includes the signaldetector, transmitter and controller.
 10. The device of claim 8, whereinthe controller is capable of re-enabling the transmitter upon receipt ofthe authorization signal.
 11. The device of claim 8, further comprising:a timer capable of counting for the specified period of time; andwherein the controller determines if the authorization signal has beenreceived at the device prior to the timer expiring at the specifiedperiod of time.
 12. The device of claim 11, wherein the authorizationsignal is received at the device, and wherein the timer restarts tocount for the specified period of time, and the controller permits thedevice to transmit information via the transmitter to the base stationupon receipt of the authorization signal.
 13. The device of claim 12,wherein the controller further permits the transmission of informationvia the transmitter to the base station upon receipt of theauthorization signal until expiration of the specified period of time onthe timer and non-receipt of a second authorization signal at thedevice.
 14. The device of claim 11, wherein the controller disables thetransmitter providing that the specified period of time on the timer hasexpired.
 15. The device of claim 11, wherein the transmitter is enabledfor the transmission of information to the base station providing asecond authorization signal has not been received at the device and thespecified period of time has not expired on the timer.
 16. The device ofclaim 8, wherein the device and the base station communicate with eachother over a radio communication channel.
 17. The device of claim 8,wherein the device and the base station communicate with each other inaccordance with a Global system for Mobile Communications (GSM)protocol.
 18. An apparatus for authorizing a user terminal tocommunicate with a base station in a communication system, the userterminal including a transmitter for transmitting information to thebase station, the apparatus comprising: means for determining if anauthorization signal has been received from the base station at the userterminal within a specified period of time for the transmission of theauthorization signal, the authorization signal authorizing the userterminal to communicate with the base station; and means for disablingthe transmitter of the user terminal providing that the authorizationsignal has not been received within the specified period of time. 19.The apparatus of claim 18, further comprising: means for re-enabling thetransmitter of the user terminal upon receipt of the authorizationsignal.
 20. The apparatus of claim 18, wherein the means for determiningif an authorization signal has been received at the user terminal withina specified period of time, further comprises: means for starting atimer to count for the specified period of time; and means fordetermining if the authorization signal has been received at the userterminal prior to the timer expiring at the specified period of time.21. The apparatus of claim 20, further comprising: means for receivingthe authorization signal at the user terminal; means for restarting thetimer to count for the specified period of time; and means forpermitting the user terminal to transmit information via the transmitterto the base station upon receipt of the authorization signal.
 22. Theapparatus of claim 21, wherein the means for permitting the userterminal to transmit information further comprises: means for permittingthe user terminal to transmit information via the transmitter to thebase station upon receipt of the authorization signal until expirationof the specified period of time and non-receipt of a secondauthorization signal.
 23. The apparatus of claim 20, wherein means fordetermining if the authorization signal has been received at the userterminal prior to the timer expiring at the specified period of time,further comprises: means for providing a signal to disable thetransmitter of the user terminal providing that the specified period oftime on the timer has expired; and means for disabling the transmitterof the user terminal.
 24. The apparatus of claim 20, wherein the meansfor determining if the authorization signal has been received at theuser terminal prior to the timer expiring at the specified period oftime, further comprises: means for permitting the transmission ofinformation from the transmitter of the user terminal to the basestation providing it is determined that a second authorization signalhas not been received and the specified period of time on the timer hasnot expired.
 25. The apparatus of claim 18, wherein the apparatuscomprises a modem including a software component having software runningthereon and a hardware component including the means for determining andthe means for disabling.